The mechanisms by which Ti additions catalyse the nucleation of β-Sn are studied in 550 µm Sn-3Ag-0.5Cu (wt%) solder balls and joints on Cu and Ni substrates. It is shown that at least two new intermetallic compounds (IMCs), Ti 2 Sn 3 and (Ti,Fe,Cu)Sn 2 , form as a result of a 0.2wt% Ti addition. The nucleation potential of each IMC was studied by electron backscatter diffraction (EBSD) of tin droplets solidified on the cross sectioned facets of each IMC. It is found that reproducible orientation relationships (ORs) form only between β-Sn and Ti 2 Sn 3 and that the two ORs generate good atomic matching between the Sn atoms in Ti 2 Sn 3 and the closest packed plane in β-Sn, {100}. β-Sn cyclic twinning occurred in droplets on Ti 2 Sn 3 where the twinning axis <100>Sn was always parallel with the lowest disregistry direction in the ORs. In solder balls and joints, the Ti 2 Sn 3 addition triggered up to 12 independent β-Sn grains, whereas Ti-free SAC305 always solidified with one independent grain.

The microstructure of Sn-Ag-Cu (SAC) solder joints plays an important role in the reliability of electronics, and interlaced twinning has been linked with improved performance. Here, we study the three-dimensional (3-D) shape of interlaced regions in Sn-3.0Ag-0.5Cu (SAC305) solder balls by combining serial sectioning with electron backscatter diffraction. In solder balls without large Ag3Sn plates, we show that the interlaced volume can be reasonably approximated as a hollow double cone with the common 〈100〉 twinning axis as the cone axis, and the 〈110〉 from all three twinned orientations making up the cone sides. This 3-D morphology can explain a range of partially interlaced morphologies in past work on 2-D cross-sections.